You did not provide enough details to determine which of many possible problems may have happened. Perhaps MfgTool did not see a kindle attached in USB Downloader mode because you had successfully switched it to fastboot mode just like you told it to do...

You are right, I think MFgTool did not see a Kindle attached. What maybe the reasons.....?
I already switched Kindle to USB recovery mode. Open MFgTtool and press fastboot mode and nothing is happening...